Microstructural development and mechanical properties of hypereutectic Sn–Cu solderalloys

TLDR: In this paper, a transient directional solidification system was used, which permitted the assessment of microstructures along castings lengths of hypereutectic Sn-2.0 and 2.8% Cu alloys, for a wide spectrum of experimental tip growth rates (V L ) and tip cooling rates (T • ).

Abstract: In this study a transient directional solidification system was used, which permitted the assessment of microstructures along castings lengths of hypereutectic Sn–2.0 wt% and 2.8 wt% Cu alloys, for a wide spectrum of experimental tip growth rates ( V L ) and tip cooling rates ( T • ). The primary Cu 6 Sn 5 IMC developed typical M-shaped or H-shaped morphologies. However, rod-like particles prevailed along the Sn-rich β matrix of both alloys evaluated. Smaller inter-branch spacings ( λ ) are shown to be associated with the alloy having higher Cu content, and the presence of Cu 3 Sn (e) intermetallic compound (IMC) was detected by X-ray diffraction (XRD). Such IMC developed probably due to the combination of high cooling rates during solidification and incomplete peritectic reaction along the Cu-enriched regions of the casting, which provided less η than predicted under equilibrium conditions. Hall–Petch type relationships are proposed interrelating Vickers hardness (HV), ultimate tensile strength ( σ u ) and elongation to fracture against λ . The two alloys examined showed roughly similar trends considering HV and σ u , despite having a very different ductility behavior. However, the mechanical strength of each alloy is shown to be controlled by different microstructure features. The higher fraction of eutectic rod-like Cu 6 Sn 5 intermetallics observed for the Sn–2.0 wt% Cu alloy was shown to be associated with the corresponding higher experimental mechanical strength.